Perfluoroelastomers have achieved outstanding commercial success and are used in a wide variety of applications in which severe environments are encountered, in particular those end uses where exposure to high temperatures and aggressive chemicals occurs. For example, these polymers are often used in seals for aircraft engines, in semiconductor manufacturing equipment, in oil-well drilling devices, and in sealing elements for industrial equipment used at high temperatures.
The outstanding properties of perfluoroelastomers are largely attributable to the stability and inertness of the copolymerized perfluorinated monomer units that make up the major portion of the polymer backbones in these compositions. Such monomers include tetrafluoroethylene (TFE) and perfluoro(alkyl vinyl)ethers (PAVE). In order to develop elastomeric properties fully, perfluoroelastomers are typically crosslinked, i.e. vulcanized. To this end, a small percentage of cure site monomer is copolymerized with the perfluorinated monomer units. Cure site monomers containing at least one nitrile group, for example perfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene are especially preferred. Such compositions are described, for example, in U.S. Pat. Nos. 4,281,092; 4,394,489; 5,789,489; and 5,789,509.
The polymerization processes of perfluoroelastomers are most typically done in the presence of a perfluoro carboxylic acid salt or fluorinated sulfonic acid salt. If the salt contains a metal ion, it contaminates the formed polymer. If the salt is a non-metal, usually the resulting pH of the polymerization media is acidic leading to corrosion of polymerization vessel or downstream lines and vessels, and subsequent contamination of the resulting polymer. Further, coagulation of the emulsion or dispersion is usually accomplished by use of magnesium, barium, or other metallic salts resulting in two distinct problems. First, they add metallic contamination to the elastomeric crumb and second, the metallic salts of the perfluoro carboxylic acids become much more difficult to remove from the crumb.
The prior art further teaches compounding the perfluoroelastomer, for example, on a roll mill, Banbury mixer, extruder, or the like. In this step, crosslinking catalysts or other additives may be mixed with the perfluoroelastomer crumb in the melt to facilitate sufficient crosslinking as may be required by the application. For example, one goal may be to attain sufficient crosslinking to achieve good high temperature compression set resistance. Compounding may actually result in the addition of metallic and/or other contaminants by the direct addition via additives; additionally high temperature melt compounding often results in metal contamination by corrosion of the compounding equipment and exposure to environmental contamination. If organic crosslinking agents are used, the resulting articles are usually brown due to thermal decomposition of the agents.
Perfluoroelastomer articles such as seals, O-rings, and valve packings are often highly filled with carbon black or metallic fillers for reinforcement rendering them opaque and providing an additional source of contamination. When exposed to plasmas in end uses such as semiconductor manufacturing, the polymeric component of these articles is etched away, leaving the fillers as undesirable particle contaminants. Furthermore, as the polymer decomposes any fillers such as metals, metal oxides or metal salts originally contained in articles may be released.
Recent patents of Saito et al. and Coughlin and Wang (U.S. Pat. No. 5,565,512, and WO 02/48200) have discussed the value of producing clear and pure perfluoroelastomer parts with low metal ion contamination. Market forces that are driving the move to clear, clean perfluoroelastomer parts include both the semi conductor industry and the pharmaceutical industry which desires extremely low concentrations of metals. In addition, the pharmaceutical and biotechnology industries desire overall purity and elimination of certain perfluoro carboxylic acids which accumulate in the body is highly desirable. For example, some companies manufacturing fluoropolymer resins or parts have established limits of perfluoro octanoic acid (PFOA), the acid form of ammonium perfluoro octanoate (APFO) which is a common surfactant used in fluoromonomer emulsion polymerization.
However, the need for crosslinkable perfluoroelastomers and crosslinked parts that have a low metallic ion contamination and a low perfluoro carboxylic concentration has not been met with the usual processes of forming these. Therefore, one embodiment of the present invention is a method for producing perfluoroelastomer compositions having low metallic ion contamination and low perfluoro carboxylic concentration.